Physical activity (PA) has multiple physical and mental health benefits during youth, but most adolescents do not meet PA guidelines. In most studies, girls are less active than boys, and girls' PA declines dramatically during adolescence. The purpose of the Trial of Activity for Adolescent Girls (TAAG) is to test the effectiveness of a multi component school-based and community-linked intervention to prevent the decline in PA and cardiopulmonary fitness in girls. TAAG is a collaborative multi-center study, and the present team from two universities in San Diego, CA, submits this proposal for a Field Center. The team has extensive experience in virtually all areas of the proposed study. We propose intervention and measurement methods, but the final methods will be based on Steering Committee decisions and pilot studies in Phase 1. Each Field Center will recruit six "sets" consisting of middle schools (grades 6-8) with at least two nearby community agencies. "Sets" matched on demographic characteristics will be randomly assigned to intervention or control conditions. Approximately 140 sixth grade girls will be recruited from each middle school and will be measured in the 6th through 9th grades. A multi component intervention is proposed that is consistent with an ecological model of behavior change. School-based interventions will include: improved physical education (PE), more supervision for PA throughout the school day, and health education modules supplemented with homework, parent materials, and a computer-based assessment and behavior change program. Community agencies will be assisted to increase PA programs for girls at their facilities and at schools. Mentors and web-based chat rooms will provide targeted and ongoing social support for girls. Girls' input will be sought in the development and formative testing of all program components. Primary outcome measures collected each Spring are proposed as the PWC170 fitness test, the CSA accelerometer activity monitor, and a validated self-report of PA. Secondary outcomes will be assessed by direct observation of PE and leisure settings, anthropometrics, school records, and student self-report. Assessment of hypothesized intervention mediators and quality of implementation of all intervention components will enhance understanding of intervention effects.